NEW YORK - At least 25 countries around the world block Web sites for political, social or other reasons as governments seek to assert authority over a network meant to be borderless, according to a study out Friday.

The actual number may be higher, but the OpenNet Initiative had the time and capabilities to study only 40 countries and the Palestinian territories. Even so, researchers said they found more censorship than they had initially expected, a sign that the Internet has matured to the point that governments are taking notice.

"This is very much the revenge of geography," said Rafal Rohozinski, a research fellow at the University of Cambridge in England.

China, Iran, Myanmar, Syria, Tunisia and Vietnam had the most extensive filters for political sites. Iran, Oman, Saudi Arabia, Sudan, Tunisia, the United Arab Emirates and Yemen had the strictest social-filtering practices, blocking pornography, gambling and gay and lesbian sites.

In some countries, censorship was narrow. South Korea, for instance, tends to block only information about its neighboring rival, North Korea.

Yet researchers found no filtering at all in Russia, Israel or the Palestinian territories despite political conflicts there.

Governments generally had no mechanism for citizens to complain about any erroneous blocking, with Saudi Arabia, Oman and the United Arab Emirates being among the exceptions.

The OpenNet Initiative, a collaboration between researchers at Cambridge, the University of Oxford, Harvard University and the University of Toronto, has previously published reports detailing censorship in specific countries. The latest study was its attempt to compare filtering worldwide.

The study did not attempt to chronicle the effectiveness of the efforts. Some technical approaches are better than others in blocking sites, but all can be bypassed with enough technical know-how to use "proxy" techniques or special software. The organization said the regions chosen for review should not be considered comprehensive. It didn't include any countries in North America or Western Europe on grounds that filtering practices there have been better known than elsewhere. It also excluded North Korea and Cuba for fear of risks to collaborators it would need in those countries. The group supplied software to volunteers in each of the countries tested. Web sites checked include those for gambling, pornography and human-rights abuses. Jonathan Zittrain, professor of Internet governance and regulation at Oxford, said filtering appeared to occur most widely in countries where Internet penetration is higher, possibly explaining the lack of any censorship efforts in Russia and Egypt.